1. Field of the Invention
The present invention relates to a machine tool for precision machining of a workpiece, and to a bed structure of such a precision machine tool.
2. Description of the Related Art
A conventional precision machine tool is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 10-151534. As shown in FIG. 1A, in the disclosed machine tool, a machining unit, which includes a tool T provided on a main spindle 204 having a horizontal rotation axis, is provided on a Z-axis unit 203 (movable along a horizontal Z-axis). The Z-axis unit 203 is supported on an X-axis unit 202 (movable along a horizontal X-axis perpendicular to the Z-axis), which is disposed on a bed 201.
A C-axis unit 207 having a horizontal C-axis is disposed in opposition to the main spindle 204. The C-axis unit 207 supports a workpiece W for rotation about a horizontal rotational axis. The C-axis unit 207 is supported on a B-axis unit 209 (rotatable about a vertical B-axis), which is supported on a Y-axis unit 210 (movable along a vertical Y-axis), which is disposed on the bed 201.
A point of the workpiece W to be machined (hereinafter referred to as a “work point”) is moved or indexed to a predetermined position by means of the C-axis unit 207, the B-axis unit 209, and the Y-axis unit 210, whereas a machining point of a tip end of the tool T is moved or indexed to a predetermined position by means of the X-axis unit 202 and the Z-axis unit 203, whereby the work point of the workpiece W is machined (cut or ground) by the tool T at its machining point.
In the conventional machine tool, the position of the work point of the workpiece W, which is represented by “A” in FIG. 1A (overall front view), is separated by a “distance Lbw” from the B-axis. Therefore, if an error α is generated as shown in FIG. 1B (partial plan view) when the B-axis unit 209 is rotated by an angle θ from a position (indicated by broken lines) at which the C-axis coincides with the Z-axis, in order to index the work point, the work point deviates from its theoretical position “A(θ)” to a position “A(θ+α).” When the tool T is moved toward the position “A(θ)),” which deviates from the actual position “A(θ+α),” the tool T machines the position “A(θ),” although the position to be machined at that time is “A(θ+α).” Such an error becomes remarkable as the “distance Lbw” increases. Further, in addition to the error involved in position indexing, an error stemming from a positioning deviation at the time of B-axis stoppage becomes remarkable as the “distance Lbw” increases.
Moreover, in the conventional machine tool, as shown in FIG. 1C (partial front view), the position “A” of the work point is separated by a “distance Lyw” from the Y-axis. Therefore, when a ram 217 (movable member) of the Y-axis unit is vertically moved from a position at which the position “A” of the work point coincides with the tip end of the tool T, in order to machine the work point A, vertical forces Fu and Fd stemming from machining resistance are applied to the work point A. The ram 217 is held by a nut 221 in screw-engagement with a ball screw 220. Stemming from the “distance Lyw” and the “forces Fu and Fd,” a moment is generated (an unnecessary stress acts on the nut 221 in a direction not coinciding with the Y-axis), whereby the ram 217 may incline as shown on the right side in FIG. 1C. When an “error β” is generated stemming from the inclination, the work point deviates from its theoretical position “A” to a position “A(β).” When the tool T is held at a height corresponding to that of the position “A,” which deviates from the actual position “A(β),” the tool T machines the position “A,” although the position to be machined at that time is “A(β).” Such an error becomes remarkable as the “distance Lyw” increases.
Influence of these errors is at a level which can be ignored in machine tools which perform ordinary machining. However, in precision machine tools which perform machining with very high accuracy on the order of several hundreds to several tens of nanometers, influence of such errors is large, and such errors must be suppressed.
Incidentally, a bed used in a precision machine tool such as a grinding machine is generally formed by casting. In general, such a bed is cast to have a hollow structure in such a manner that the bed is reinforced by integrally formed ribs arranged in a grid pattern. Further, a plurality of holes as cast (hereinafter referred to as “cast holes”) penetrate the side and bottom walls of the bed. The reason why the bed is cast to have a hollow, rib-reinforced structure is to reduce the weight of the bed and the influence of long-term distortion of the material. The cast holes cannot be eliminated, because they are essential for casting a bed having a hollow, rib-reinforced structure.
In some cases, instead of a cast bed, a bed formed of stone such as granite is used in a super-precision machine tool which must machine optical components or the like with very high machining accuracy. Such a bed formed of stone such as granite has characteristics such that the bed exhibits smaller long-term changes in material properties and a larger heat capacity as compared with the case of cast beds, and generally has a solid structure.
The conventional cast bed is prone to receive influence of outside air temperature, because the inner structure of the bed is exposed to the outside air through the cast holes, and the area of contact with the outside air is larger than in a case of a bed having a solid structure.
In general, when an object has a temperature difference with respect to outside air temperature, the time from exposure to outside air temperature until the object attains the same temperature as the outside air temperature decreases as the ratio of surface area S to volume V; i.e., S/V, increases. FIG. 14 shows results of calculation for obtaining temperature changes of three objects which have the same volume and the same temperature difference with respect to outside air temperature, but have different surface areas. These three objects are formed of the same material (gray cast iron), and the calculation for each object was performed for the case where the initial temperature is 25° C., and the ambient temperature is 20° C. FIG. 14 shows that a spherical object, having the smallest S/V value, takes the longest time to attain the outside air temperature, and that the time required to attain the outside air temperature decreases as the S/V value increases. In other words, influence of outside air temperature increases as the S/V value increases.
Since the conventional cast bed has a hollow, rib-reinforced structure, the bed has an S/V value greater than that of a bed having a solid structure. Therefore, the bed temperature is prone to change as the outside air temperature changes, and affects structures mounted on the bed; specifically, slide surfaces, the tool spindle, and the workplace spindle, whereby an error is produced in the positional relation between a workplace and a tool. As a result, machining accuracy fluctuates in the course of long-term machining.
The above-described problem exerts considerable influence not only on a machine tool disposed in a place, such as an ordinary plant, where the outside air temperature changes greatly, but also on a machine tool, such as a super precision machine tool, which is placed in a thermostatic room, whose interior temperature is controlled to a set temperature ±1° C., and which is required to provide very high machining accuracy.
Meanwhile, the conventional bed formed of stone such as granite has a larger heat capacity as compared with the case of cast beds, and has a smaller area of contact with the outside air, because it assumes the shape of a solid rectangular parallelepiped. Therefore, the conventional bed formed of stone such as granite has an advantage in that the temperature of the bed is unlikely to follow changes in the outside air temperature, and the bed enables machining with high accuracy. However, the granite is more expensive than a casting, and the degree of freedom in design is low, because machining of granite is difficult.